It is of importance to increase the efficiency of operations on multiple work pieces which are similar to each other, and are clamped on a work piece carrier. Such work pieces can be made in small, intermediate or large series in computer numerically controlled (CNC) work centers, or can be worked on with multi-spindle special machines. The effective operating time of such CNC operating centers and special machines should be effectively increased, so that the cost of machining, per work piece, drops. At the same time, costs can be saved by use of work pieces which will form components of other machines, and which are as much as possible already in the form of elements ready to be assembled, for example being injection molded, or pressed. Such work pieces should require only minimum fine working involving material removal. That means that such work pieces frequently have to be worked on at only a few regions, so that the individual work times for working on the work pieces are relatively short. However, other time periods then become even more important in such working. These auxiliary or lateral time periods are those required for supplying the work pieces, clamping them, re-clamping them in a different position, if necessary, and removing the work pieces. These auxiliary time periods form a cost factor which becomes increasingly important as the initial preparation of the work pieces themselves becomes more accurate requiring little re-working.
CNC operating centers and special machinery are highly developed; this permits to fulfil the highest requirements of quality and accuracy. To insure that the precision possible by the machines can also reflect itself in the work piece itself, it is necessary that the work piece is positively fixed on the work piece carrier, and reliably clamped thereon in a fixed position. Frequently it is possible to meet the highest requirements and quality only by fine machining the work pieces once they have been clamped so that small errors which might arise due to re-positioning and re-clamping the work pieces are avoided.
Various different work clamping arrangements for CNC operating centers and special machinery are known in engineering practice; these clamping arrangements are suitable for small, intermediate or large series operations and permit work pieces to be commonly clamped and worked on. The work pieces are supplied in a predetermined quantity and placed in a predetermined arrangement. In one type of such a multiple clamping arrangement, a work piece carrier in the form of a beam of generally square cross-section is provided. The beam is rotatable about its longitudinal axis and securely retained in bearing blocks at both ends thereof. On one side it is coupled to a rotary indexing table. Each of the four sides of the work piece carrier have a clamping surface for the reception of, for example, four work pieces located in a row. The rotatable table permits indexing the work piece carrier about its longitudinal axis over a predetermined angle to bring the work pieces, clamped in rows on the clamping surfaces in suitable receptors, to a desired operating position where they can be worked on. In the operating position, the work pieces are in properly aligned position in relation to the respective operating spindles and tools. These clamping arrangements permit, however, only working of the work pieces on three sides. Work pieces which require working thereon on more than three sides have to be re-positioned and re-clamped for, for example, finishing work in a further subsequently connected CNC operating center. Re-clamping of the work pieces is time-consuming and, as mentioned, introduces the danger of errors in the positions of the work pieces after re-clamping.
Multiple clamping arrangements have also been proposed in order to permit operating on four or five sides of work pieces in one clamping operation. An elongated work piece carrier has up to four round indexing tables located in a row adjacent each other. The work piece carrier itself is rotatable about its longitudinal axis and suitably supported for such rotation. The indexing tables receive the work pieces from a loading and unloading station. They are then rotated into a working position, so that they can be worked on at a working station and, after termination of the work on the work pieces, they are rotated back for reception by the loading/unloading station. The indexing tables are coupled to a common drive system which permits placement of work pieces, clamped on the tables, at the actual operating station into the respectively required different positions with respect to the tools themselves. These clamping arrangements permit, however, only simultaneous clamping of a highly limited number of work pieces. The indexing range of the tables is usually limited to less than 300 circular degrees. These arrangements also require, usually, an individual drive and additional axial controls so that they cannot be combined with existing CNC operating centers or special machinery without substantial rebuilding and reconstruction. Efficient, economic working on the work pieces is possible only to a limited extent. For this reason, working on more than three sides of the work pieces is usually not done and it was preferred to return to three-sided work pieces in order to be able to clamp a larger number of work pieces simultaneously. The disadvantage thereof, that the work pieces must be re-positioned and re-clamped at least once, is accepted in these clamping arrangements which work with work piece carriers in beam shaped, or cube shaped, work piece carriers and clamping arrangements, as above briefly described.